1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a method and an apparatus for supporting handover between cells while a User Equipment (UE) performs packet reordering.
2. Description of the Related Art
A Universal Mobile Telecommunication Service (UMTS) system is a 3rd Generation (3G) asynchronous mobile communication system, which uses wideband Code Division Multiple Access (CDMA) and is based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), which are European mobile communication systems. The UMTS system is an example of cellular mobile communication systems, each of which includes a plurality of cells divided from an entire service area of the system and supports handover between cells in order to guarantee continuity of communication.
When an active UE moves from one cell (source cell) to another cell (target cell), the target cell establishes a communication path for the UE and re-establishes entities of each layer related to the established communication path. Especially, when the communication of the UE supports Automatic Retransmission reQuest (ARQ), it may sometimes become necessary for the target cell to re-establish an ARQ entity for the UE. As used here, handover re-establishing an ARQ entity refers to handover in which the source cell removes an existing ARQ being used therein and the target cell sets a new ARQ entity.
When handover re-establishing an ARQ entity is performed in a conventional mobile communication system, a higher layer entity of the ARQ entity performs accumulative retransmission. The accumulative retransmission can reduce the complexity of the higher layer entity although packets already transmitted in the source cell may be retransmitted in the target cell. In a conventional UMTS mobile communication system, the handover re-establishing an ARQ entity occurs during Serving Radio Network Subsystem (SRNS) reallocation during which a serving Radio Network Controller (RNC) of a UE is changed. Because the SRNS reallocation does not frequently occur, the accumulative retransmission is performed in consideration of complexity rather than efficiency.
Meanwhile, in the 3rd generation Partnership Project (3GPP), which is in charge of standardization of the UMTS, active discussion about Long Term Evolution (LTE) of the UMTS system as an evolved mobile communication system of the UMTS system is ongoing. The LTE is technology, which is targeting commercialization thereof by the year 2010 and the realization of high speed packet-based communication at a speed of about 100 Mbps. To this end, various schemes are being discussed, which include a scheme for reducing the number of nodes located in a communication path by simplifying the structure of a network, and a scheme for approaching a wireless protocol to a wireless channel as much as possible.
In an evolved mobile communication system such as the LTE system, an ARQ entity is located in a Node B that is a lower layer entity of the RNC. Therefore, an ARQ entity is always re-established in handover among Node Bs, and thus the handover re-establishing an ARQ entity occurs much more frequently in an evolved mobile communication system, such as the LTE system, than the UMTS system. Therefore, it is necessary to develop technology for improving communication efficiency in the handover re-establishing an ARQ entity in an evolved mobile communication system, such as the LTE system.